AbstractImproving cotton (Gossypium spp.) heat tolerance (ability to set bolls in high temperature environments) has been an important aspect of the American Pima (Pima) cotton (G. barbadense L.) breeding program for over 25 years. However, heat tolerance is difficult to quantify. This report estimates change in heat tolerance of Pima cotton through yield response over 30 years in six Arizona counties that differ in elevation and mean summer temperatures. Pima cotton/upland cotton (G. hirsutum L.) lint yield percentages were compared for each county by regression with year of production. County mean yield percentages were also compared with elevations. Pima cotton yields as percentages of upland cotton yields increased in five of the six counties over the 30‐year period. Pima lint yield increased from 57 % of upland yield in 1956 to 75 % in 1985 when averaged over the six counties. This increase represents an estimated 206 kg lint ha‐1 in addition to any upland yield increase. Comparison of regression coefficients suggest that nearly 50 % of the 30‐year lint yield increase of Pima cotton at lower elevations was the result of increased tolerance to high temperature in improved cultivars. County 30‐year mean Pima cotton lint yields as percentage of upland cotton yield increased significantly (P < 0.001) from 52.2% to 82.1 % as county elevations increased from 37 m to 1273 m (cooler summer temperatures). These results support reports of greater heat tolerance for upland cotton than Pima cotton, but also show that the difference between species has been reduced substantially by breeding in the last 30 years.

Cotton ( Gossypium hirsutum L.) planting seed density has been related to seedling emergence and vigor, although its effects on yield are not clearly understood. Seed lots of the cultivar ‘DP90’ were separated into four density classes using a gravity separation table. These seed classes were evaluated along with an undivided seed sample and a sample from processed seed selected for commercial use. The seed classes were planted at three planting rates in seven tests conducted from 1983 to 1985 at Maricopa, Marana, and Safford, AZ. The soils at these locations are Typic Camborthids, Thermic Torrifluvents, and Typic Torrifluvents, respectively. The lowest density seed had significantly lower standard and cold germination test results; however, there were no differences among the other five seed classes in 2 of the 3 yr. Field emergence of the highest density seed was significantly greater than that of the other density classes for the seven tests combined, and was significantly lower for the lowest density seed. Lint yields produced by plants from the highest density seed were significantly greater than those of the four other seed classes, and the lowest density seed produced significantly lower yields. There were no significant interactions between planting seed density and planting rate for yield. The results indicate that cotton planting seed quality could be improved slightly by selecting higher density seed

Delayed cotton (Gossypium spp.) planting generally reduces lint yield because it reduces the length of the growing season. Planting cotton too early also can reduce lint yield. Most growers believe that reduced yield with early planting is caused by reduced stand. Previous research suggested that low temperature during germination and early seedling growth may reduce lint yield regardless of stand. This study attempts to clarify low temperature effects on lint yield by partitioning cotton lint yield reduction from early planting into the effects from reduced stand and other factors. Soils were Typic Torrifluvents. Thermic Torrilluvents, and Anthropic Torrifluvents. Upland cotton (Gossypium hirsutum L.) had significantly (P < 0.05) reduced lint yield from early planting in five tests. American pima cotton (G. barbadense L.) grown in four of the tests had smaller, nonsignificant lint yield decreases from early planting in three of the tests and higher lint yield from early planting in the fourth test. Early planted cotton generally was shorter. A comparison of upland cotton lint yields, using adjustment by regression analyses with plant population, indicated that something other than plant population, presumably low temperature, caused approximately 100, 94, 66, 22, and 0 % of the lint reduction from early planting in five analyses. Thus physiological and morphological effects of low temperatures early in the cotton planting season often may contribute as much to reduced lint yield as does reduced stand.

Plant populations giving the highest lint yield of American pima cotton (Gossypium burbdense L.) grown in 1-m spaced rows change as plant heights change. Therefore, plant height should be considered in future plant population studies with pima cotton. This study was conducted to determine the number and type of plant height measurements needed to give a satisfactory estimate of mean plant height for a plot. Plant height uniformity data were obtained from 12 plots (four locations with three plots per location) in Arizona. Soils were Torrifluvents Haplostolls, Anthropic Torrifluvents, and Thermic Torrifluvents. Measurement of four to eight plants per plot, half of the number on each end, appeared to be adequate to characterize the plant height of a plot. Plant heights were normally distributd. A systematic selection produced lower variation (about onehalf as many plants needed to be measured) but over-estimated mean plant height by an average of 12.5%.

AbstractRapid cotton (Gossypium hirsutum L.) boll development and opening can be beneficial in late season. It can result in increased lint yield, earlier harvest, higher grade, less insecticide use, and reduced storm damage. Research was conducted in the field on a fine loamy, mixed (calcareous), hyperthermic, Anthropic Torrifluvent soil. This study evaluates the effects of water stress in late season (early irrigation termination) and pink bollworm [Pectinophora gossypiella (Saunders)] infestation on time required for cotton bolls to develop from flower to open boll in the field. Water stress speeded boll opening. Bolls from stressed cotton were obtained from field tests with five dates of final irrigation. They opened an average of 17 days earlier than bolls of unstressed cotton in late season. Boll period averages ranged from 41 to 76 days depending upon the degree of water stress and time of year. On droughty (shallow) soil [fine loamy, mixed (calcareous) hyperthermic Anthropic Torrifluvent], water stress effects were significant, but smaller. Delayed boll opening may explain reduced lint yield which at times is associated with late final irrigation. Pink bollworm infested bolls opened about 3 days earlier than non‐infested bolls in two tests with deep soil, but only 0.5 days earlier in a test on droughty soil. Boll period population resembled a normal distribution around the mean until 93% of the bolls were open. For a given day of flowering, 80 % of the bolls (from 10 to 90 %) opened within a 8‐ to 11‐day period.

AbstractSubstantial mid‐ and late‐season topping of American Pima cotton (Gossypium barbadense L.) plants occurs in field plantings. Lygus bug (Lygus hesperus Knight) feeding is the apparent cause of topping. The effects of mid to late‐season topping on lint yield and several yield components of ‘Pima S‐4’ cotton were examined. Topping of Pima S‐4 cotton in mid‐July, early‐August, and mid‐August reduced plant height, reduced the number of main stem nodes, increased boll set on top fruiting branches, and resulted in additional branch nodes on top fruiting branches. Plants topped 17 July produced 300%, 100%, and 60% more bolls on the first, second, and third branch, respectively, below the point of topping than did the check plants. The effect was less when topping was later in the season. The yield patterns in lower branches were not different from the checks. Topping did not affect: 1) lint yield (3 years), 2) number of bolls set, 3) seed/boll, 4) lint/boll, 5) lint/seed, 6) seed weight, 7) days to boll maturity, or 8) pink bollworm infestation. Fruit set was not delayed by topping. Fruiting points, which aborted on normal plants, produced bolls on topped plants up to the potential for the cultivar and environment. This has important implications for studies on insect damage to plants and for efforts to increase lint yield by reducing boll abscission.

Large differences in vegetative regrowth among nine cultivars were noted in an American Pima cotton (Gossypium barbadense L.) regional test after severe hail damage on 22 July 1974. These differences were correlated negatively with the lint yield of the same cultivars in a nearby, undamaged, Pima cotton regional test. Correlations of number of new leaves and dry weight of new leaves 5 weeks after hail with lint yield on the undamaged test were −0.84 and −0.91, respectively. Theories on transport and utilization of photosynthate in cotton were used to explain differences in recovery among cultivars.

AbstractThe relationship of number of seed/boll to other yield components and fiber quality of two cultivars of Pima cotton (Gossypium barbadense L.) was investigated as part of a study of the effects of environment on the variability of these cultivars in three tests. A fourth test had only one cultivar. Bolls examined contained from 1 to 31 seed. Environment of the four tests strongly influenced the proportion of bolls in the different seed number classes. Plants grown under favorable production conditions, such as lower summer temperatures, productive soil, and a wet irigation regime, produced a higher percentage of large bolls (more seed) than those grown less favorable environments. Cultivars did not differ in seed/boll. An increase in the number of seed/boll was associated with the following: decreased mean seed wt, increased lint wt/seed, decreased fiber length and uniformity, and increased micronaire, but strength was unaffected. The unexpected increase in lint wt/seed with a decrease in seed size can be explained by the observed concomitant increase in fiber coarseness.

Growers have been planting American Pima cotton ( Gossypium barbadense L.) in skip‐row patterns without supportive research data on skip‐row effects on yield and quality of Pima cotton. The purpose of this research was to obtain such data on skip‐row planting for Pima cotton. Pima cotton was grown under irrigation in five skip‐row patterns in randomized blocks in each of 5 years. All skip‐row patterns increased lint yields on a planted‐area basis as compared to solid planted. Skipping single rows increased lint yield of a two‐row strip 20%. Skipping two rows increased lint yield an additional 8%. In plant‐6‐skip‐2 pattern, lint yield of outside rows was increased 28% over that of the center two rows. The second rows from the outside had 5% increase in lint yield. A tall cultivar benefited more from skip‐row than did a short cultivar. Skip‐row effect on outside rows was greater during years of high lint yield than during years of low yield (r = 0.90). In these tests the skip‐row yield increases were about 60% of those reported on Upland cotton ( G. hirsutum L.). Outside rows and single rows had lower gin turn‐out, more trash in lint, and slightly higher lint percent. Skip‐row patterns had no effect on fiber quality.

AbstractThe purpose of this study was to determine if two border rows on each side of yield rows were necessary to prevent border effects in irrigation tests on American Pima cotton (Gossypium barbadense L.). Individual rows were harvested in 6‐row plots on first harvest (67% to 89% of total harvest) in Pima cotton irrigation, cultivar, plant‐population tests. Tests were conducted for 4 years. Analyses of these data indicate that lint yield of outside‐border rows (rows 1 and 6 of 6‐row plots) was significantly affected by adjacent irrigation treatment for the three methods of irrigation tested. Border effects on lint yield of inside‐border rows (rows 2 and 5) were much lower, but significant at the 0.05 or 0.50 level. Difference in lint yield between inside‐border rows and yield rows (rows 3 and 4) ranged from 2.7 to 4.7% for the three irrigation methods.Need for more than one pair of border rows is indicated. However, border effects on inside‐border rows were small in most comparisons. In one instance, absolute yields of inside‐border rows were affected by the adjacent treatments, but relative yield levels of different irrigation treatments were not affected.

AbstractCotton (Gossypium sp.) growers in recent years have been changing from planting cotton by drill‐planting and thinning to planting to a stand. Much planting to a stand is done by hill‐dropping seed. This study was initiated to compare lint yield of ‘Pima S‐4’ cotton (G. borbadense L.) when drill‐planted and thinned and when hill‐dropped at 23‐cm spacing. There were three population levels, 1, 2 and 3 plants/hill, for hill‐dropped and comparable plant populations for drill‐planting and thinning. There were no significant differences in lint yield between hill‐dropping and drill‐planting in 3 years.